Radio frequency (RF) power amplifiers are often used in portable battery-operated wireless devices, such as cellular telephones. Extending battery life is a concern for users and manufacturers of these battery-operated wireless devices. One of the factors in determining the battery life in such devices is the power consumption of the RF power amplifiers. The RF power amplifiers are designed to operate into a target load impedance and are typically coupled with an antenna of the battery-operated wireless device.
However, under an antenna mismatch condition, e.g., when the antenna approaches another object (e.g., metal structures, human contact, or the like), the load impedance of the RF power amplifier changes, and the RF power amplifier draws excess current. In some cases, the current can exceed two times the current drawn under a target load impedance. When the RF power amplifier draws excess current, the battery life of the battery-operated wireless device is reduced. In addition, the adjacent channel power ratio (ACPR) and error vector magnitude (EVM) limits are often exceeded when the RF power amplifier draws excess current.
Current-limiting circuits have been designed to limit the current draw of an RF power amplifier under an antenna mismatch condition. These current-limiting circuits rely upon a current-sensing circuit to determine the instantaneous current draw of an RF power amplifier. However, present current-sensing circuits suffer from accuracy problems due to their components having characteristics that vary with process and temperature. This variation may reduce current-sensing accuracy by as much as thirty percent.